1. Field of the Invention
The present invention relates to techniques for producing thin films on substrates by vapor deposition processes, and more specifically, it relates to a method for producing multi-component thin films with precise gradients in chemical composition in either or both the vertical and horizontal directions.
2. Description of Related Art
Multilayer coatings are formed by depositing alternating layers of two (or more) materials of different refractive index. In the ideal case, these two materials form stable structures with sharp, abrupt interfaces. The profile of an ideal interface can be represented by a step function. The chemical composition changes abruptly in the vertical direction (the growth direction) from one layer to the other. Realistically however, because of the diffusion of one material into the other, the interfaces are never abrupt and can be better described as a chemically graded region where the concentration of the individual constituents varies continuously. The chemical gradient of the interface depends on growth parameters, which are deposition process specific, and hard to control.
It is desirable to form a controlled chemical gradient in multilayer coatings. Chemical gradients can be formed in either the vertical direction (e.g., within one layer, on the interface between two layers, etc.) or in the lateral direction (e.g., varying the chemical composition across the surface). However, this is very difficult, especially when the gradient has to be formed with sub-nanometer thickness (vertical case) or has to vary precisely across large areas (lateral case).